1. Field of the Invention
The present invention relates to a semiconductor device able to improve a mobility of carriers, and an EL (electroluminescent) display device, a liquid crystal display device, and a calculating device utilizing the semiconductor device.
2. Description of the Related Art
There are various kinds of display apparatuses in the related art, and CRT (cathode-ray tube) displays, liquid crystal displays, and EL (electroluminescence) displays are typical ones. The CRT displays are widely used so far because of their high display quality and lower manufacturing cost. Nevertheless, a CRT display suffers from the difficulties of reducing the size and lowering the electric power consumption of the cathode-ray tube. For these reasons, demand for the liquid crystal displays and the EL displays rapidly increases. Especially, because of such features as self-emitting, low driving voltages, and high brightness, more and more expectations are placed on the organic EL display devices used in the EL displays.
The liquid crystal displays are widely used because of their good visibility and fast responding performance. Meanwhile, the EL displays, which exhibit good visibility and responding performance, are also being widely spread gradually. Further, the liquid crystal displays and the EL displays both have lower power consumption than that of the CRT displays.
In the above displays, one of the commonly used active devices is a transistor including an organic semiconductor layer, a source electrode (first electrode), a drain electrode (second electrode), and a gate electrode (third electrode). FIG. 27 and FIG. 28 are sectional views each showing a TFT (Thin Film Transistor), specifically, a thin film FET (Field Effect Transistor) of the related art.
In the thin film FET shown in FIG. 27, a source electrode 203 and a drain electrode 204 are separated by an electrically-neutral organic semiconductor layer 205 (channel region), and are arranged in the same plane. The gate electrode 201 is electrically separated from the organic semiconductor layer 205 by a gate insulating layer 202.
In the thin film FET shown in FIG. 28, a source electrode 213 and a drain electrode 214 are arranged to be separated at a distance in the same plane on an organic semiconductor layer 215. The gate electrode 211 is electrically separated from the organic semiconductor layer 215 by a gate insulating layer 212.
In these thin film FETs of the related art, the organic semiconductor layer can be formed by coating, so it is possible to largely reduce the fabricating cost. However, there exists a problem in that the charge mobility of an organic semiconductor material is much lower than that of an inorganic semiconductor material. In recent years, the organic semiconductor material Polythiophene is found to be of high charge mobility (Applied Physics Letter, vol. 69. p 4108(1996)), and therefore is attracting attention, but its charge mobility is less than 0.1 (cm2/V·sec) about one order of magnitude lower than that of the current amorphous silicon. Furthermore, the switching speed of an active device using the organic semiconductor material generally is of the order of KHz, and is not suitable for driving devices for displaying fine motion pictures.
In the related art, in order to increase the switching speed of an active device, in addition to improving the mobility of an organic semiconductor material, attempts have also been made in shortening the channel length of the active device. It was considered that the current flowing in the semiconductor layer would increase if the channel length was shortened, and thus the organic thin film transistor (TFT) could be used to drive an EL display. However, in order to pattern a channel length (that is, the source-drain distance) less than a few μm, complicated photolithography processing is needed, and thus the fabricating costs goes up.
To solve this problem, the static induction transistor (SIT), in which a source electrode, a gate electrode, and a drain electrode are stacked vertically in sequence, was presented in the ninth lecture course of Bioelectronics Division of Applied Physics Association (2001).
FIGS. 29A and 29B are sectional views showing static induction transistors (SIT) of the related art. FIG. 29A shows a state in which a voltage is not applied, and FIG. 29B shows a state in which a voltage is applied and a depletion region is formed. As shown in FIGS. 29A and 29B, in a SIT active device of the related art, a source electrode 223, an organic semiconductor layer 225, and a drain electrode 224 are stacked vertically in sequence, and in the center portion of the organic semiconductor layer 225, several rod-shaped gate electrodes 222 are formed in a row at certain intervals and approximately in parallel with the source electrode 223 and the drain electrode 224. With this compound organic EL transistor, as shown in FIG. 29B, if a gate voltage is applied to the gate electrode 222, the depletion region in the organic semiconductor layer 225 expands, and the resistance between the source electrode 223 and the drain electrode 224 increases, thereby the current between the gate electrode 222 and the drain electrode 224 is switched ON or OFF. With such an SIT, the area of the active device can be reduced.
As shown in FIGS. 29A and 29B, because the channel length of the SIT can be controlled by using the film thickness of the semiconductor layer, it is possible to form a thin semiconductor layer by coating or other film fabrication techniques, so the SIT is expected to work as a transistor capable of fast response in the future.
Turning to the problem addressed by the present invention, however, in the above SIT, when a gate voltage is not applied to the gate electrode 222, a current flows between the source electrode 223 and the drain electrode 224; when a gate voltage is applied to the gate electrode 222, the current between the source electrode 223 and the drain electrode 224 is switched off (that is, such a SIT is a normally ON type transistor).
Further, with the current film fabrication technique, the film thickness of the organic semiconductor layer 225 is thicker than that of the organic semiconductor layer 215 in the TFT of the related art as shown in FIG. 28, and as a result, the channel length between the source electrode 223 and the drain electrode 224 in the above SIT is shorter than that of the TFT of the related art. Therefore, in the SIT, the resistance between the source electrode 223 and the drain electrode 224 decreases, and there arises a problem in that the current in an OFF state is large.